MINOR: regex: Change the struct containing regex

This change permits to remove the typedef. The original regex structs
are set in haproxy's struct.
diff --git a/src/pattern.c b/src/pattern.c
index 6d7de52..3ac8f03 100644
--- a/src/pattern.c
+++ b/src/pattern.c
@@ -476,7 +476,7 @@
 /* Parse a regex. It is allocated. */
 int pat_parse_reg(const char **text, struct pattern *pattern, int *opaque, char **err)
 {
-	regex *preg;
+	struct my_regex *preg;
 
 	preg = calloc(1, sizeof(*preg));